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  • 1. Gundale, Michael J.
    et al.
    Bach, Lisbet H.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nordin, Annika
    The impact of simulated chronic nitrogen deposition on the biomass and N-2-fixation activity of two boreal feather moss-cyanobacteria associations2013In: Biology Letters, ISSN 1744-9561, E-ISSN 1744-957X, Vol. 9, no 6, p. 20130797-Article in journal (Refereed)
    Abstract [en]

    Bryophytes achieve substantial biomass and play several key functional roles in boreal forests that can influence how carbon (C) and nitrogen (N) cycling respond to atmospheric deposition of reactive nitrogen (N-r). They associate with cyanobacteria that fix atmospheric N-2, and downregulation of this process may offset anthropogenic Nr inputs to boreal systems. Bryophytes also promote soil C accumulation by thermally insulating soils, and changes in their biomass influence soil C dynamics. Using a unique large-scale (0.1 ha forested plots), long-term experiment (16 years) in northern Sweden where we simulated anthropogenic Nr deposition, we measured the biomass and N-2-fixation response of two bryophyte species, the feather mosses Hylocomium splendens and Pleurozium schreberi. Our data show that the biomass declined for both species; however, N-2-fixation rates per unit mass and per unit area declined only for H. splendens. The low and high treatments resulted in a 29% and 54% reduction in total feather moss biomass, and a 58% and 97% reduction in total N-2-fixation rate per unit area, respectively. These results help to quantify the sensitivity of feather moss biomass and N-2 fixation to chronic Nr deposition, which is relevant for modelling ecosystem C and N balances in boreal ecosystems.

  • 2. Gundale, Michael J.
    et al.
    From, Fredrik
    Bach, Lisbet H.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nordin, Annika
    Anthropogenic nitrogen deposition in boreal forests has a minor impact on the global carbon cycle2014In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 20, no 1, p. 276-286Article in journal (Refereed)
    Abstract [en]

    It is proposed that increases in anthropogenic reactive nitrogen (N-r) deposition may cause temperate and boreal forests to sequester a globally significant quantity of carbon (C); however, long-term data from boreal forests describing how C sequestration responds to realistic levels of chronic N-r deposition are scarce. Using a long-term (14-year) stand-scale (0.1ha) N addition experiment (three levels: 0, 12.5, and 50kgNha(-1)yr(-1)) in the boreal zone of northern Sweden, we evaluated how chronic N additions altered N uptake and biomass of understory communities, and whether changes in understory communities explained N uptake and C sequestration by trees. We hypothesized that understory communities (i.e. mosses and shrubs) serve as important sinks for low-level N additions, with the strength of these sinks weakening as chronic N addition rates increase, due to shifts in species composition. We further hypothesized that trees would exhibit nonlinear increases in N acquisition, and subsequent C sequestration as N addition rates increased, due to a weakening understory N sink. Our data showed that understory biomass was reduced by 50% in response to the high N addition treatment, mainly due to reduced moss biomass. A N-15 labeling experiment showed that feather mosses acquired the largest fraction of applied label, with this fraction decreasing as the chronic N addition level increased. Contrary to our hypothesis, the proportion of label taken up by trees was equal (ca. 8%) across all three N addition treatments. The relationship between N addition and C sequestration in all vegetation pools combined was linear, and had a slope of 16kgCkg(-1)N. While canopy retention of N-r deposition may cause C sequestration rates to be slightly different than this estimate, our data suggest that a minor quantity of annual anthropogenic CO2 emissions are sequestered into boreal forests as a result of N-r deposition.

  • 3.
    Högberg, Mona N.
    et al.
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Blaško, Róbert
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Holm Bach, Lisbet
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Hasselquist, Niles J.
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Egnell, Gustaf
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Näsholm, Torgny
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Högberg, Peter
    Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    The return of an experimentally N-saturated boreal forest to an N-limited state: observations on the soil microbial community structure, biotic N retention capacity and gross N mineralisation2014In: Plant and Soil, ISSN 0032-079X, E-ISSN 1573-5036, Vol. 381, no 1-2, p. 45-60Article in journal (Refereed)
    Abstract [en]

    To find out how N-saturated forests can return to an N-limited state, we examined the recovery of biotic N sinks under decreasing N supply. We studied a 40-year-old experiment in Pinus sylvestris forest, with control plots, N0, three N treatments, N1-N3, of which N3 was stopped after 20 years, allowing observation of recovery. In N3, the N concentration in foliage was still slightly elevated, but the N uptake capacity of ectomycorrhizal (ECM) roots in N3 was no longer lower than in N0. Per area the amount of a biomarker for fungi, here mainly attributed ECM, was higher in N3 and N0 than in N1 and N2. Retention of labeled (NH4)-N-15 (+) by the soil was greater in the control (99 %) and N3 (86 %), than in N1 (45 %) and N2 (29 %); we ascribe these differences to biotic retention because cation exchange capacity did not vary. Gross N mineralisation and retention of N correlated, negatively and positively, respectively, with abundance of ECM fungal biomarker. The results suggest a key role for ECM fungi in regulating the N cycle. We propose, in accordance with plant C allocation theory, that recovery is driven by increased tree below-ground C allocation to ECM roots and fungi.

  • 4. Maaroufi, Nadia I
    et al.
    Nordin, Annika
    Hasselquist, Niles J
    Bach, Lisbet H
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Gundale, Michael J
    Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils2015In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 21, no 8, p. 3169-3180Article in journal (Refereed)
    Abstract [en]

    It is proposed that carbon (C) sequestration in response to reactive nitrogen (N-r) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While studies have helped clarify the magnitude by which N-r deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50kgNha(-1)yr(-1)) in the boreal zone of northern Sweden to understand how atmospheric N-r deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of similar to 15% and a significant increase of similar to 30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10kgCkg(-1)N. We also found a concomitant decrease in total microbial and fungal biomasses and a similar to 11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26kgCkg(-1)N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.

  • 5. Maaroufi, Nadia, I
    et al.
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bach, Lisbet H.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bokhorst, Stef
    Liess, Antonia
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Rydberg Laboratory of Applied Science, School of Business, Science and Engineering, Halmstad University, Halmstad, Sweden.
    Gundale, Michael J.
    Kardol, Paul
    Nordin, Annika
    Meunier, Cedric L.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung (AWI), Biologische Anstalt Helgoland, Helgoland, Germany.
    Nutrient optimization of tree growth alters structure and function of boreal soil food webs2018In: Forest Ecology and Management, ISSN 0378-1127, E-ISSN 1872-7042, Vol. 428, p. 46-56Article in journal (Refereed)
    Abstract [en]

    Nutrient optimization has been proposed as a way to increase boreal forest production, and involves chronic additions of liquid fertilizer with amounts of micro- and macro-nutrients adjusted annually to match tree nutritional requirements. We used a short-term (maintained since 2007) and a long-term (maintained since 1987) fertilization experiment in northern Sweden, in order to understand nutrient optimization effects on soil microbiota and mesofauna, and to explore the relationships between plant litter and microbial elemental stoichiometry. Soil microbes, soil fauna, and aboveground litter were collected from the control plots, and short- and long-term nutrient optimization plots. Correlation analyses revealed no relationships between microbial biomass and litter nutrient ratios. Litter C:N, C:P and N:P ratios declined in response to both optimization treatments; while only microbial C:P ratios declined in response to long-term nutrient optimization. Further, we found that both short- and long-term optimization treatments decreased total microbial, fungal, and bacterial PLFA biomass and shifted the microbial community structure towards a lower fungi:bacterial ratio. In contrast, abundances of most fungal- and bacterial-feeding soil biota were little affected by the nutrient optimization treatments. However, abundance of hemi-edaphic Collembola declined in response to the long-term nutrient optimization treatment. The relative abundances (%) of fungal-feeding and plant-feeding nematodes, respectively, declined and increased in response to both short-term and long-term treatments; bacterial-feeding nematodes increased relative to fungal feeders. Overall, our results demonstrate that long-term nutrient optimization aiming to increase forest production decreases litter C:N, C:P and N:P ratios, microbial C:P ratios and fungal biomass, whereas higher trophic levels are less affected.

  • 6. Palmroth, Sari
    et al.
    Holm Bach, Lisbet
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nordin, Annika
    Palmqvist, Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nitrogen-addition effects on leaf traits and photosynthetic carbon gain of boreal forest understory shrubs2014In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 175, no 2, p. 457-470Article in journal (Refereed)
    Abstract [en]

    Boreal coniferous forests are characterized by fairly open canopies where understory vegetation is an important component of ecosystem C and N cycling. We used an ecophysiological approach to study the effects of N additions on uptake and partitioning of C and N in two dominant understory shrubs: deciduous Vaccinium myrtillus in a Picea abies stand and evergreen Vaccinium vitis-idaea in a Pinus sylvestris stand in northern Sweden. N was added to these stands for 16 and 8 years, respectively, at rates of 0, 12.5, and 50 kg N ha(-1) year(-1). N addition at the highest rate increased foliar N and chlorophyll concentrations in both understory species. Canopy cover of P. abies also increased, decreasing light availability and leaf mass per area of V. myrtillus. Among leaves of either shrub, foliar N content did not explain variation in light-saturated CO2 exchange rates. Instead photosynthetic capacity varied with stomatal conductance possibly reflecting plant hydraulic properties and within-site variation in water availability. Moreover, likely due to increased shading under P. abies and due to water limitations in the sandy soil under P. sylvestris, individuals of the two shrubs did not increase their biomass or shift their allocation between above- and belowground parts in response to N additions. Altogether, our results indicate that the understory shrubs in these systems show little response to N additions in terms of photosynthetic physiology or growth and that changes in their performance are mostly associated with responses of the tree canopy.

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